The overall objective of this project is to determine what role the oocyte plays in the fundamental organization, differentiation and function of the ovarian granulosa cells. Granulosa cells of large antral follicles are heterogeneous in their expression of specific phenotypic characteristics. One population of granulosa cells, the mural granulosa cells, is associated with the basal lamina and the other, the cumulus cells, with the oocyte. Such microenvironmental associations suggest that the oocyte and the basal lamina both influence the fate and function of the cells associated with them. Although many factors originating in the pituitary gland, the follicular theca or even in the granulosa cells themselves participate in granulosa cell development and function, the overall goal of this project is to determine what role the oocyte plays in the development and function of the granulosa cells. The hypothesis underlying this work is that the selection of the specific pathway for development, as functional cumulus cells or as mural granulosa cells, is determined by the association of the granulosa cells with either the oocyte or the basal lamina. Furthermore, it is hypothesized that the oocyte is actually the dominant determinative factor in granulosa cell development. More specifically, the default program of granulosa cell differentiation is proposed to yield the mural granulosa cell phenotype, which is augmented by contact with basal lamina, while paracrine factors from the oocyte suppress the expression of this phenotype and promote the cumulus cell phenotype. The following specific aims will address this hypothesis. First, mRNA species expressed specifically by mural and cumulus granulosa cells will be identified and cloned. These will be used as markers of the cumulus and mural granulosa cell phenotypes. Specific Aims 2 and 3 will compare the ability of oocyte secretions and components of basal lamina to affect these phenotypes. It will be determined whether the development of the phenotypic characteristics of mural granulosa cells is suppressed by paracrine factors from oocytes even when the expression of the mural granulosa cell phenotype is promoted by components of basal lamina (Aim 2). Then, it will be determined whether microsurgical removal of the oocyte from oocyte-cumulus cell complexes promotes the expression of the mural granulosa cell phenotype and the loss of the cumulus cell phenotype. If so, it will be determined whether paracrine factors from oocytes maintain the cumulus cell phenotype and suppress the expression of the mural granulosa cell phenotype even when oocytectomized complexes are maintained in contact with basal lamina in vitro (Aim 3). Completion of these aims will further the understanding of how the oocyte influences the fundamental organization of the follicle. This may be by a dominant influence of the oocyte in creating the heterogeneity in the pattern of gene expression and cellular function of granulosa cells. These new perspectives on follicular development could lead to novel approaches to the regulation of fertility and the resolution of ovarian dysfunction by means that target oocyte-somatic cell interactions.